Hi,
Could you clarify slightly.
Do you mean, install Exchange in a child domain?
If so, yes, you can.
You need to run Forest Prep on a DC in the root of the forest and then run domain prep in Root and any domain which will host Exchange objects.
Cheers
Nathan
Nathan Winters - MVP Exchange Server
MCSE & MCSA 2000 & 2003 + Messaging, MCITP Exchange 2007, MCP, VMWare VCP v2 & v3.
Welcome to the Microsoft Messaging and Mobility User Group: http://www.mmmug.co.uk